1. Field of the Invention
The present invention relates to a printing method and a printing apparatus for producing an image on a printing medium and more specifically to a printing method and a printing apparatus used in a case of performing so-called multipass printing.
2. Description of the Related Art
An ink jet printing method, because of its variety of advantages such as low noise, low running cost and ease with which to reduce an apparatus size and to produce color images, has found a wide range of applications as in printers, copying machines and the like. Most of such printing apparatuses employs a print head, which integrates a plurality of printing elements (i.e., nozzles for ejecting ink in a case of an ink jet printing method), to improve the printing speed.
The printing apparatus that uses a print head with the plurality of arrayed printing elements is known to produce a stripe like density variation that appears periodically in a subscan direction (the direction in which the printing medium is fed) and may become one cause of deterioration of printing quality. These periodic stripe like density variations are very conspicuous. Possible causes of these stripe like density variations include, in the case of the ink jet printing system, variations in an amount of ink ejected and an ink ejection direction among nozzles, deviations between paper feed and nozzle pitch, and density variations caused by time variations among each scanning operation.
To reduce the stripe like density variations and improve the print quality, a variety of methods have been disclosed.
For example, Japanese Patent Application Publication No. 59-31949 (1984) discloses a method which eliminates the stripe like density variation (hereinafter, simply referred to "a boundary stripe") that occurs at a boundary (hereinafter, simply referred to "joint") between respective areas, each of which is printed with respective plurality of scans in a main scan direction (hereinafter, simply referred to as "scans"). In the method, pixels at the lower end of the area printed at a previous scan and pixels at an upper end of an area printed at a current scan are overlapped, and then the overlapped pixels are selectively printed with these two times of scans.
A well-known conventional method for realizing further improvement of print quality is a divisional printing method (multipass printing method). The divisional printing method will be explained as follows.
In a case of a print head founded on an ink jet system, the print head with a plurality of nozzles may have a slight manufacturing error among the nozzles during a process of manufacturing the print head. Such error results in variations in the amount of ink ejected and in the ejection direction among the nozzles on printing, which in turn forms the stripe like density variations on a printed image, thus degrading the print quality. One example of such causing of the stripe like density variation is shown in FIGS. 1A-1C. FIG. 1A shows that, in the print head with 8 ink ejection nozzles, the volumes of ink ejected from the nozzles and the ink ejection directions differ from one nozzle to another. If the printing is performed using a head having such ink ejection characteristic variations among nozzles, ink dots formed will vary in a size and a landing position among rasters printed respectively corresponding to respective nozzles, as shown in FIG. 1B. As a result, a blank portion may be formed at a central part of FIG. 1B as shown or conversely a portion may be formed where dots are overlapped more than necessary. FIG. 1C shows a density distribution on an image printed with such dots. These density variations are recognized as the stripe like density variations that may become the cause of deterioration of the print quality.
On the other hand, the divisional printing method (multipass printing method), rather than printing all pixels in a single pass (or in one scan) of the print head in the main scan direction, prints them in a plurality of scans by using different nozzles in different scans.
FIGS. 2A-2C explain the multipass printing that uses the same head as used in the method shown in FIGS. 1A-1C. As shown in FIG. 2A, with respect to the printing area shown in FIG. 1B, three scanning operations of the print head are performed, and then, printing each half of this area, which is covered by four vertically arranged pixels, is completed with two scanning operations. In this case, the eight nozzles of the print head are divided into two groups, i.e., upper four nozzles and lower four nozzles. The dots formed with one nozzle in one scan is what conforms to data obtained by thinning the image data of each half area to approximately one-half according to a predetermined method. Then, by feeding a paper at a distance corresponding to four pixels, nozzles different from those used for the first printing face the same printing position and complementarily form dots according to the remaining half thinned data, thus completing the printing. According to this printing method, since one raster (one line of dots in a main scan direction) is printed with inks ejected from different nozzles (in the example shown, two different nozzles), the influence of variations among nozzles can be alleviated and the density variations can be reduced as shown in FIGS. 2B and 2C.
This divisional printing method divides image data so that respective image data for the first and second scans are extracted according to respective predetermined rules and the extracted data complement each other. Generally, the extraction is performed using mask processing and a most popular mask is a checker pattern mask where the data is extracted vertically and horizontally at every other pixels like a checker pattern as shown in FIGS. 3A-3C. In a unit printing area (in this case, a 4-pixel unit area), the printing is completed with two scans (2 passes), where at the first scan, a checker pattern as shown in FIG. 3A is printed, and at the second scan, an inverse checker pattern as shown in FIG. 3B is printed.
In the multipass printing method described above, increasing a number of divisions (passes) is effective in further improving the print quality. For example, when a 2-pass printing and a 10-pass printing are compared, the 2-pass printing completes one raster by using two different nozzles whereas the 10-pass printing completes the same raster by using 10 different nozzles. Hence, the degree to which the printed result is affected by the ejection characteristic variations among nozzles is relatively smaller in the 10-pass printing than in the 2-pass printing, and then the overall influence of the characteristic variations on the print quality becomes smaller in the 10-pass printing to that extent. As a result, the stripe like density variations is made inconspicuous.
Regarding the multipass printing method described above, Japanese Patent Application Laid-Open No. 6-143618 (1994) discloses a method which considers a fact that a total numbers of scans applied for completing printing of both side areas of the joint or the boundary portion are greater than the number of scans required to complete printing in other areas, and reduces the print duty in both side areas of the boundary portion, particularly when a printing medium that quickly absorbs ink is used to improve the print quality of the multipass printing method.
In any of the multipass printing methods described above, when there is a discrepancy between the paper feed amount and the nozzle pitch of the print head, the so-called boundary stripe is unavoidably produced. This phenomenon will be explained by referring to FIGS. 4A and 4B.
FIGS. 4A and 4B represent examples of the 2-division printing (2-pass printing) for a case where an 8-nozzle head is used as in FIGS. 3A-3C. For simplicity of explanation, it is assumed that the head has no ejection characteristic variations among these 8 nozzles. Further, the mask used for the division printing is checker pattern mask. That is, during odd-numbered scan the checker pattern mask is used and, during even-numbered scan the inverse checker mask is used, so as to complete the printing in each divided area with a combination of these two different kinds of scans.
FIG. 4A shows a result of printing when the paper is fed at a distance exactly corresponding to four pixels in each of the two paper feed operations. For simplicity, only dots printed during the first and third scans are shown at a right of the drawing and the dots printed during the second scan is not shown. A lower end portion of the area printed at the first scan adjoins an upper end portion of the area printed at the third scan, and the boundary between these areas constitutes the joint between both areas printed during respective scans. As shown in FIG. 4A, when the paper is fed at the distance exactly corresponding to eight pixels in two paper feed operations, the printed dots at the joint between the areas of the scans are arranged correctly and no boundary stripe is formed.
On the other hand, FIG. 4B shows a result of printing that is performed when the paper feed amount is greater than the required four-pixel distance. As similar to FIG. 4A, at the right of the drawing, only the dots printed at the first and third scans are shown. The paper is fed two times from the first to the third scan and because each paper feed amount is greater than the distance corresponding to four pixels, the paper is fed at a distance greater than eight pixels between the first scan and the third scans. Thus, a group of dots printed at the third scan is spaced from the group of dots printed at the first scan by a distance corresponding to the paper feed amount error. This space or gap between the areas of the scans is recognized as a white boundary stripe on a printed image, which deteriorates the printed quality.
It should be noted that conversely, when the paper feed amount is smaller than the required distance so that the paper feed error appears as a minus amount, the gap is recognized as a black boundary stripe. Further, although the above phenomenon is explained based only on the paper feed errors assuming that there are no errors in the pitch among nozzle groups, there may be cases where the overall nozzle pitch is larger or smaller than a normal nozzle pitch. In these cases, the black or white boundary stripes may appear, respectively.
Moreover, even when a paper feed error averaged over the entire printing medium is zero, if there is any two times of paper feed whose total feed amount is larger or smaller than the distance corresponding to eight pixels in paper feeds for the entire printing, the same phenomenon as described above may occur at the boundary between the areas of the two times of scans. The amount of paper feed may vary depending on errors of the outer diameter of paper feed rollers, the environment in which the printing is performed, and the kind and state of the printing medium such as paper. It is therefore difficult to totally eliminate errors produced at every feeding operation.
As described above, the boundary stripe at boundaries or joints may occur also in the multipass printing system. Increasing the number of passes is conducive to alleviating the boundary stripes. Because an increase in the number of passes decreases the number of dots printed at one scan, the boundary stripes, even if produced owing to the paper feed errors, decrease in density.
The degree to which the boundary stripes are conspicuous to the human eye depends upon a spatial frequency of the stripe like density variations. More specifically, stripes of the same density may or may not become noticeable depending on their intervals.
This characteristic is shown in a graph of FIG. 5, in which an ordinate axis represents an intensity of sensitivity and an abscissa axis represents a spatial frequency (cycles/mm). As can be seen from the figure, the spatial frequency realizing the highest sensitivity exists around 1.1 cycles/mm, which means that the stripes at 0.9 mm intervals show most conspicuously. When a print head with 8 nozzles arranged at 600 dpi pitch is used, the interval at which the stripes are produced at the multipass printing is as follows. In comparing between respective printed images produced in the 2-pass printing and the 10-pass printing, the 2-pass printing feeds the paper at a distance corresponding to a 40 nozzle pitch and the interval of the stripe occurrence is a 40 nozzle pitch, which is equal to 1.69 mm (25.4 mm/600 dpi.times.40 pixels). In other words, the spatial frequency is 0.59 cycles/mm. On the other hand, in the 10-pass printing, the paper is fed at a distance corresponding to 8 nozzles and the interval of stripe occurrence is an 8-nozzle pitch, which is equal to 0.338 mm (25.4 mm/600 dpi.times.8 pixels). Thus the spatial frequency is 3.0 cycles/mm. FIG. 5 apparently shows that the interval of stripe occurrence in the 10-pass printing is less conspicuous than that in the 2-pass printing.
However, increasing the number of passes reduces the print area to be completed in one pass or one scan, which in turn increases a time required to print the entire image. For example, increasing the number of passes from the 2-pass printing to the 10-pass printing increases the printing time by five times, based on simple calculation.